In many of today's data processing systems, including Local Area Networks (LAN's), it is often desirable to have devices manufactured by different vendors communicate to each other within the network. Typically, each device is connected to a transmission medium such as a main bus of the network, either directly or through an interface component. Before transmission, outgoing messages from a sending device are formatted in a manner which is acceptable by the receiving device. Upon receipt of the transferred data, the receiving device disassembles the message and acknowledges that the message has been successfully transferred.
In order to achieve such communications however, it is necessary for the devices to follow the same communications throughout the network, or at least throughout the shared bus. Currently there exists several Media Access Control protocols (MAC's) for device transmission or communication over a shared network bus. One such protocol requires the device to listen for no activity across the bus before transmitting signals on the bus. After transmission, the device listens for responses to the transmission to determine the failure or success of the transmission. Failure may occur where two or more devices attempt to transmit simultaneously.
Another protocol, employed by Ethernet (TM), follows the access method known as Carrier Sense Multiple Access with Collision Detection (CSMA/CD). The CSMA/CD method uses the principle of "listen before transmitting, listen while transmitting" to gain access to the network. Each device on the network listens to the bus and does not transmit if the bus is in use. If a bus detects silence, it is able to transmit. There is still the possibility of simultaneous transfers which cause failure of transmissions.
A token passing protocol (e.g. IEEE 802.4 Standard) enables devices on the network to share access without the possibility of simultaneous transfers which cause transmission failure. In this protocol, a bus token provides permission to a device to transmit signals over the bus. The token is passed from one device to another around the network in an established order which forms a logical ring (not necessarily a physical ring). Only the device with the token is allowed to transmit, thus eliminating collisions during normal operations. This protocol results in a network that is highly controllable, predictable and stable under all communication loads.